A tire enhanced in traction performance and brake effect by forming the zigzag shaped main groove on the tread surface in the circumferential direction is known. It is also proposed to enhance the wet resistance performance and steering stability of a tire in the intermediate and terminal phase of the wear of the tread surface, by defining the deflection width of zigzag at the groove bottom greater than the deflection width at the tread surface, in the main groove, so as to reduce the track wear.
Furthermore, in order to prevent uneven wear of a tire for heavy duty vehicles without lowering the wet grip performance, various proposals such as the Japanese Patent Publication No. 51-15282, Japanese Laid-open Patents Nos. 46-4553, 61-92902, 61-178205, 61-115703, and the U.S. Pat. No. 4,423,760 have been disclosed. But none of them proposes radical countermeasures. For example, in the Japanese Patent Publication No. 51-15282, the main groove dividing the center ribs is designed so as to decrease the width of the center ribs toward the inside in the tire radial direction, that is, the sectional shape of the main groove is designed substantially in a reverse V-form. And also to prevent biting of pebbles, a projecting part is provided in the main groove, whereby the track wear generated in the ribs is decreased. In this case, however, the groove shape is complicated, and there are manufacturing problems. Moreover pebbles may be caught in both side slits in the grooves, and the rib side edges may be broken. On the other hand, as shown in FIG. 9 (a), (b), the wear is excessive in the external angle portion of the zigzag groove, because the rigidity of the external angle portion is lower than that of other parts. Therefore, the external angle portions are worn partially and progressively. As the method of enhancing the rigidity in the external angle portion so as to reduce the uneven wear, as shown in FIG. 9 (c), it is proposed to remove the external angle portion. That is, to form the main grooves G3, G4 by peripheral groove components Ga, Gc parallel to the tire peripheral direction, and oblique groove components Gb, Gd inclining at an angle of .alpha. to the tire circumferential direction.
By this method, the reduction of track wear may be achieved, but from the viewpoint of grip performance of the tire, it is necessary to increase the angle .alpha. to the circumferential direction of the oblique groove components Gb, Gd, which may sometimes result in the formation of heel-and-toe wear which is often found in the block pattern, near the ridge of the oblique groove components.
As proposed above, when the deflection width of zigzag at the groove bottom is defined larger than that at the tread surface, the track wear of tire is slightly decreased. But the wear still occurs, and in the terminal phase of wear of the tread surface, the wet grip force is extremely lowered, and the brake effect when traveling on a wet road is not perfect.